<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>test</title>
<style>
.btn {
width: 100px;
text-align: center;
height: 50px;
line-height: 50px;
background: #000;
color: #fff;
position: relative;
}
.btn:after {
position: absolute;
content: '';
width: 30px;
height: 30px;
background: #fff;
top: 10px;
right: -15px;
border-radius: 100%;
}
</style>
</head>
<body>
<div class="btn">
button
</div>
</body>
</html>
test
button